Different types of systems are known that detect if a setpoint temperature is exceeded. In particular, there are indicators described in many documents reporting a failure of the cooling system or indicators showing that a product temperature has been too high.
French Patent Publication No. FR 2 899 684 discloses a product preservation status indicator to detect if the temperature of a product has exceeded a predetermined threshold called the setpoint temperature. This indicator comprises two transparent films hermetically assembled so as to trap two doses of liquid or viscous products, these doses being separated by a glue joint hermetically sealing adjacent faces of the two films. This glue is such that it loses its adhesive properties when a predetermined temperature is exceeded, so that if the setpoint temperature is reached after the indicator has been activated, then the viscous products will be free to mix and the result of this mix will be an irreversible change in the state such as a change in the color. Apart from the complexity of the system, this indicator cannot be used to make a visual check so as to immediately make sure that the state of preservation of the product has been good and maintained between two temperature thresholds. This indicator can only detect a crossing of a temperature threshold, but it is impossible to know if the product was too cold or too hot. Furthermore, the status change of the system is irreversible, and this is not a color memory system. Finally, irreversibility of the system makes it impossible to reuse the indicator once the temperature threshold has been exceeded.
European Patent Publication No. EP 1 410 368 discloses a fuel tank provided with a safety label located on the outside face of a tank, said label containing at least one reversible thermochromic material. This label changes color above a temperature threshold to inform the user about a possible burn on the skin if he does not have any protection. The label simply detects that the temperature has been exceeded (overheating). Therefore this label cannot be used as an indicator for a low temperature crossing (frost), for example for products that must not be frozen under any circumstances.
Indicators according to the state of the art have disadvantages firstly due to their complexity, and secondly because they are limited to a single threshold, in that they can indicate crossing of a temperature only above the setpoint temperature (abnormal overheating) or below the setpoint temperature (frost).
Two temperature thresholds frequently have to be monitored to assure good preservation of a product; there are many products that are sensitive to both cold and heat. Existing indicator systems are not suitable for these products. For example, some pharmaceutical and medical preparations are sensitive to cold and to heat and must not go outside a specific temperature range even for an instant. For example, a blood pouch must be kept at between +8° C. and +32° C., a vaccine must be kept (for example) between +3° C. and +8° C., and even an instantaneous exceedance of either of these two limits may make the product unsuitable for its planned used.
Similarly, most technical dispersions or emulsions (such as paints) suffer irreversible transformations as soon as they go outside a specific temperature range. And finally, many food products in the case of dispersions, emulsions, solutions or other systems, do not resist frost or excessively high temperatures; this is the case for some dairy products and some drinks (wine, beer, spirits, etc.), knowing that these products are not always stored and transported in an isothermal environment.
Thus, the problem that this invention aims to solve is to disclose a simple and reliable system for precisely determining whether or not an object or a product has gone outside a determined temperature range and to memorize this event. It must be possible to manufacture this system so that it can react to different temperature thresholds. It must be inexpensive, resistant to the environment wherein it is used and it must not contaminate the product to which it is affixed.